Only one in eight cases of child sex abuse are identified by the authorities, according to a new official new report which says five-year-olds should be given compulsory lessons on preventing paedophilia in the family.

Anne Longfield, the children’s commissioner for England, found there had been 50,000 reported cases of sex abuse over the last two years - but estimated the true number was 400,000 to 450,000.

The in-depth report said abuse in the home – by parents, siblings, other relations or trusted family friends – is a far greater problem than abuse by strangers, accounting for two-thirds of all cases.

The commissioner recommended a new emphasis on helping young children understand what types of relationships are “healthy and safe” in a bid to combat paedophilia committed by family members.

“How could it be that we can’t gamble online unless we demonstrate you are 18, but anyone can access hardcore pornography regardless of age?”

Chief Constable Simon Bailey

It came as a senior police officer called of stricter controls on online pornography because it is twisting children’s views of sex and turning them into abusers.

Ms Longfield said: “Children must be better equipped to act on the signs of child abuse.

“I want to ensure children have the confidence and resilience to understand healthy and safe relationships.

“Often they don’t have the words to describe what’s happening to them.

“They don’t have the understanding at the time whether that is a healthy relationship or not.

“We are told time and time again by teenagers and victims and survivors that if they had better information earlier, if they had help to understand relationships and help to build their confidence they would have been able to express that they were worried.”

"“A system that waits for children to tell them about something cannot be effective.”

The commissioner stopped short of setting out what lessons should include but she said they should go beyond the “mechanics” of sex to explain to children about what kinds of intimacy are appropriate.

She added: “We must also ensure that staff are tuned into identifying the signs of abuse.

“A system that waits for children to tell them about something cannot be effective.”

Chief Constable Simon Bailey, the National Police Chiefs’ Council spokesman on child protection, said youngsters are being lured into abusing siblings and other children after watching hardcore pornography online.

“I have a real concern that use of technology such as smartphones is creating a generation of children who are seeing things that children historically have never been privy to seeing,” he said.

“What they are seeing in pornography is suggesting this is normal behaviour, and it absolutely is not.

“That is … leading to wholly inappropriate behaviour and some awful, awful consequences.

“I fear peer-on-peer abuse is being driven by a normalisation of what they are viewing online.